GOES-8 Sounding derived lifted index
March 28, 2000 - Fort Worth, Texas Tornado

GOES-8 Sounder Characteristics:

18 thermal IR bands and a low-resolution visible band
8 km field of view, sampled every 10 km
patterned after HIRS (High-resolution IR Sounder)
spectral bands are sensitive to temperature, moisture and ozone
goal is to provide brightness temperatures with 1.0K absolute accuracy and 0.3K relative pecision

GOES-8 Sounder Accuracy:

Accuracy is estimated through comparison with VAS (GOES-7 VISSR Atmospheric Sounder) and radiosonde profiles
Retrieves with greater density and coverage than TOVS
Advantage - hourly retrievals

How is GOES-8 Generated?

Vertical temperature profiles are retrieved from sounder radiance measurements at 40 pressure levels
"First guess" - temperature profile is obtained from a space-time interpolation of fields provided by NWS forecast models.
Surface boundary information is obtained from hourly observations and sea surface temperatures from AVHRR.
For vertical moisture profiles, specific humidity is a differentiated quantity rather than an absolute retrieval.
Bias error is obtained by examining ensembles of collocated observations and foward calculations in vicinity of radiosondes.
Prior to calculating a retrieval, bias error for each band is subtracted from the observation.

What is a derived product?

Retrieval is processed to obtain the derived product
Image of derived product is generated by combining the sounding product with the 11 micrometer window radiance observed in the cloud-contaminated field of views
The grey scale is divided; first half represents the sounding value, whereas the second half shows the 11 micrometer cloud radiance
Color enhanced sounding portion of the grey scale to distinguish the sounding values from the cloud features

What is the lifted index?

Estimate of atmospheric stability
Represents buoyancy that an air parcel would experience if mechanically lifted from a mixed boundary layer to the 500-mb level
Expresses the difference in temperature between the ambient 500-mb temperature and the temperature of the lifted boundary layer parcel
Positive buoyancy - negative values; parcel is warmer than the environment; instability
Derived from 500-mb temperature, mean pressure, temperature and moisture for the boundary layer; all of which are available from the retrieved profile
Lifted Index values:

1 and 6	Stable Conditions, Thunderstorms Not Likely
0 and -2	Slightly Unstable, Thunderstorms Possible, With Lifting
-2 and -6	Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism
Less Than -6	Very Unstable, Severe Thunderstorms Likely With Lifting Mechanism

Fort Worth, TX tornado:

Tuesday, March 28, 2000; approximately 6:00pm CST
Downtown Fort Worth, Texas
F2 tornado, winds 113-157 mph
An area of strong convection was detected ahead of dry line, which was moving eastward across Texas. A region of instability was advected north across eastern Texas. 4.5 inch hail reported.